Dwell

As points wear, the time the two sides "dwell" together will change as there's less point material, or angled surfaces, worn lobe on the points, etc.

Somebody mentioned old dwell / rpm meters sitting on the shelf collecting dust. I've got one of those -- will sell it to your real cheap. But you'd save yourself a lot of money by just replacing points on a regular basis and forgetting about dwell tachs. The only engine I ever worked on that was easy to set dwell had a Delco distributor with the "window" that let you adjust dwell with an allen wrench. Every other kind of distributor makes setting dwell a pain. Don't worry about it, just install new points -- you'll be money ahead.

I guess I’m the guilty one for bringing this whole silly topic up. Sometimes it is just better to keep quiet, I’m a thinkin’; but, I’d better see if I can explain myself a little better.

Here's another way of thinking of dwell.

"It, in reality, is the 'unused' portion of the distributor shaft's cam".

This unused portion (along with the ‘used’ portion) can be measured electronically with a meter by hooking between the distributor lead and ground.

Looking at the rotor end of the distributor shaft with the rotor removed, the dwell angle is created by drawing a line from the point at which the rubbing block of the point arm just comes away from the cam (during rotation) in to the center of that shaft.

Then, also from the center of that shaft, draw another line back out to the point at which the rubbing block again just touches down on the distributor shaft cam.

The angle of these two connecting lines translates into dwell angle. Or, one might say, "time", if you are thinking in terms of the rotation of that shaft.

This period of time that the cam does not actually work on that rubbing block is dwell.

So, let's say you are working on an eight-cylinder engine. The distributor shaft comprises a full 360 degrees, of course; therefore, each cylinder lobe ‘segment’ gets allotted 45 degrees of the distributor shaft's rotation.

If the points are set correctly, each lobe will be lifting the point arm for a period of 7 1/2 degrees of rotation, then lowering the arm for a period of 7 1/2 degrees of rotation (points open for a total of 15 degrees). The arm will then "rest" between the two lobes for 30 degrees of that shaft's rotation.

Meanwhile, at the same time all this has been going on, the crankshaft has actually turned 90 degrees per cylinder or a total of 720 degrees per 1 revolution of that distributor shaft, because the distributor turns at one-half crankshaft speed.

I know, clear as mud. Sorry, I'm not a good talker. :>(

And, as others have already said, for these old slow-spinnin' engines, it really isn't critical to narrow the point gap down to dwell angle.

Setting the points with a feeler gauge will get you there.

But anyway, that’s the way it was figured when I was a-bein’ learnt. :>)

Allan

"Dwell" is the angle of crank rotation (measured in degrees) the ignition points are closed per distributor firing impulse. Dwell thus affects the time available for the coil primary to build up current hence how strong a spark is created when the points open. Dwell also is another way of expressing point gap presuming the distributor cam lobes are not worn. If point gap is too large, dwell angle becomes small, coil current does not build up fully and the spark is weakened. Conversely if point gap is too small, dwell angle increases and the points may not open far enough to break the primary circuit. Result is again a poor spark and burned points. As Michael suggests, dwell is pretty much meaningless at the speeds Farmall engine's run. It only becomes significant on 6 and 8 cylinder engines running faster than several thousand RPM.

Only way I know to measure dwell is with a tach-dwell meter. (Any mechanic who wrenched on cars before the advent of electronic ignition will have at least one tach-dwell meter gathering dust on a shelf in the garage…) However if you set your point gap correctly dwell will then be correct too.

Hope this makes sense...

I don't like to doing this, but I've gotta disagree with Mike here.

Dwell is the period of time that the points are closed. Yes, this is true.

But, the dwell angle is measured at the distributor shaft and is that amount of time, measured in degrees (measured at the flat face surfaces of the cam of that distributor shaft), that the distributor shaft is not "lifting" the rubbing block of the points.

Other than being mechanically driven via some of these components, it doesn't have a thing to do with valves, camshafts, crankshafts, belt pulleys, steering wheels, brake pedals, lights, steering toe-in or anything else.

All dwell is interested in doing is giving the coil enough time to electrically recharge the secondary windings of that coil. It is just another way of setting point gap.

Bottom line is: just set your points with a feeler gauge to the correct spec and you should be right in the ballpark. The dwell will automatically come in to the correct range.

Allan

Well done!!

I wish I would have had the plain common sense to hang on to those old books, as you did. :>)

Thanks,

Allan